Amphoteric: Definition and Examples in Chemistry

An amphoteric substance is one that can act as either an acid or a base, depending on the medium. The word comes from the Greek amphoteros or amphoteroi, meaning "each or both of two" and, essentially, "either acid or alkaline."

Amphiprotic molecules are a type of amphoteric species that either donates or accepts a proton (H⁺), depending on the conditions. Not all amphoteric molecules are amphiprotic. For example, ZnO acts as a Lewis acid, which can accept an electron pair from OH but cannot donate a proton.

Ampholytes are amphoteric molecules that exist primarily as zwitterions over a given pH range and have both acidic groups and basic groups.

Here are some examples of amphoterism:

• Metal oxides or hydroxides are amphoteric. Whether a metal compound acts as an acid or a base depends on the oxide oxidation state.
• Sulfuric acid (H₂SO₄) is an acid in water but is amphoteric in superacids.
• Amphiprotic molecules, such as amino acids and proteins, are amphoteric.